This invention relates to lasers and, more particularly, to a method and apparatus for producing 1.5 .mu.m band radiation efficiently by a quasi-three-level laser transition system.
Laser radiation typically is produced in a material by a three-level or by a four-level transition system. The distinction is this: In a three-level system the lower level for fluorescence is the ground level, i.e., the level with lowest energy, whereas in a four-level system the lower level lies above the ground level. Three-level systems generally are not efficient enough for practical use. To create the population inversion necessary for lasing action, one must "pump" atomic, ionic or molecular particles from one or more energy levels to higher energy levels. Since there are significantly more particles populating the ground level than higher energy levels, it is generally quite difficult in a three-level system to obtain the required energy population inversion. In a four-level system on the other hand, the lower laser energy level that is used for laser transitions typically is much higher than the ground level and therefore can be almost completely unpopulated, even at room temperature. In other words, the energy threshold to cause a population inversion at any particular temperature is lower in a four-level energy transition system than in a three-level system, resulting in a higher laser transition probability. Because of this, four-level laser transition systems are more efficient and more widely used to generate laser radiation than three-level transition systems.
"Quasi-three-level" laser transition systems are also known. For example, reference is made to the papers by L. F. Johnson, et al. which appeared in "Applied Physics Letters", Vol. 7, pp. 127-129, 1965; K. 0. White, et al., "Applied Physics Letters", Vol. 21, pp. 419-420, 1972; G. M. Zerev, et al., the "Journal of Applied Spectroscopy" (USSR), Vol. 21, pp. 1467-1469, 1974; and Tso Yee Fan, et al., "IEEE J. of Quantum Electronics", QE-23, pp. 605-612, 1987. A quasi-three-level system is one in which the lower energy state of the laser transition is close to the ground state but yet is a thermally populated state. The lower, thermally populated state generally is in a ground state manifold. In this connection, energy state manifolds are defined in a solid state lasant material by the dopant, whereas the crystalline or glass host plays a significant roll in determining the number and location of the energy levels in each of such manifolds. While quasi-three-level transitions have been observed at room temperature, generally high energy thresholds have been required in all prior arrangements to provide the necessary population inversion. This has significantly reduced efficiency.
High power 1.5 .mu.m band radiation is of particular interest in optical communications, military systems and medical systems. This wavelength is eye-safe and coincides with the low loss window of fused silica fibers. Most efforts in the past to provide radiation within the 1.5 .mu.m band, i.e., radiation having a wavelength or wavelengths falling between 1.4 and 1.6 .mu.m, have focused on the co-doping of a host crystalline or glass material. Examples of co-doping approaches to obtain this wavelength and other wavelengths can be found in U.S. Pat. Nos. 3,715,683; 4,081,761; 4,477,906; and 4,701,928. It will be recognized that a co-doping approach is inherently less efficient than one which relies on a single ion for both absorbing pumping radiation and lasing, in view of the need to provide energy transference between ions.
Other sources of 1.5 .mu.m band radiation at the time of filing this patent application are F.sup.+ -center lasers and semiconductor diode lasers. The F.sup.+ -center laser is delicate, operates at cryogenic temperatures and is not stable. While semiconductor diodes have the advantage of small size, their beam quality is not satisfactory for many applications.